Mortars, Snipers and Gunners. Mortar and Gunner can fend off most stuff. Place the Sniper where you need the most damage. Then quickly switch him where ever something comes - he will make short notice of it.
AI mode is kind of retarded. First it used 7 tries to fly for more than 3 second - then it flies forever...
To be addicting there needs to be alot more substance to the game.